INTRODUCTION

Mixing of liquid-liquid or solid-liquid system is a complex operation to analyze and

subject to many variables. The choice of mixer for a particular application depends on the degree

of bulk movement or shear mixing required by the process. In order to predict full-scale

requirements, it is usual to model the system and apply dimensional analysis.

Before the dimensional analysis can be used, three conditions must apply:

1. Geometric similarity – This will define the boundary conditions, corresponding

dimensions will have the same ratio.

2. Kinematics similarity – This requires that velocities at corresponding points must have

the same ratio ac those at other corresponding points.

3. Dynamic similarity – This requires that the ratio of forces at corresponding points is

equal to that at other corresponding points.

The modes of flow behavior exist in a mixer laminar and turbulent flow. Both these flow

conditions may be described dimensionally but for turbulent flow its behavior is less significant.

In particular, the power number becomes independent of Reynold’s number beyond a certain

turbulence range. A further factor to consider is surface waves, which are, describe by the Froude

number group. In a mixer this phenomena is usually function of the height of the vortex, which

forms. Arm field have developed a model mixer, which can be used to predict the power

consumption of a full-sized mixer by equating Reynold’s number and Froude number. The effect

of placing baffles in the mixer vessel is also investigated.

OBJECTIVES

Experiment 1 :

The objective of this experiment is to observe the flow patterns that can be achieved by the use

of different impellers with and without the use of baffles.

Experiment 2 :

The objective of this experiment is to show how the power consumed by a mixer varies with

speed, types of impeller and with the inclusion of baffles.

THEORY

An impeller is a rotating component of a centrifugal pump which transfer energy from the

motor that drives the pump to the fluid being pumped by accelerating the fluid outwards from the

center of rotation. The velocity achieved by the impeller transfers into pressure when the

outward movement of the fluid is confined by the pump casing. Impeller are usually short

cylinders with an open inlet (called an eye) to accept incoming fluid, vanes to push the fluid

radially, and splined center to accept a driveshaft. There are three types of mixing flow patterns

that are markedly different. The so-called axial-flow turbines actually give a flow coming off the

impeller of approximately 45 degree and therefore have a recirculation pattern coming back into

the impeller at the hub region of the blades.

Axial flow impellers include all impellers in which the blade makes an angle of less than

90 degree with the plane of rotation. They run at the highest efficiency and they have the lowest

NPSH requirement. They require the highest power requirement at shut off, so they are normally

started with the discharge valve open. Axial flow impellers may also be mounted near the bottom

of the cylindrical wall of the vessel.

Radial flow impellers have blades which are parallel to the axis of the drive shaft. The

smaller multiblade ones are known as turbines; larger, slower-speed impeller with two or four

blades are often called paddles. The diameter f a turbines is normally between 0.3 and 0.6 of the

tank diameter. They should be specified for high head and low flow conditions.

As we know, baffles are needed to stop the swirl in a mixing tank. Almost all the impeller

rotate in the clockwise or counter clockwise direction. Without baffles, the tangential velocity

coming from any impeller causes the entire fluid mass to spin. Most common baffles are straight

flat plate of metal that run along the straight side of vertically oriented cylindrical tank or vessel.

For unbaffles tank, there is a tendency for a swirling flow pattern to develop regardless of

the type of impeller. A vortex is produced owing to centrifugal force acting on the rotating

liquid. However, there is a limit to the rotational speed that may be used, since one the vortex

reaches the impeller, severe air entrainment may occur. In addition, the swirling mass of liquid

often generates an oscilating surge in the tank, which coupled with the deep vortex may create a

large fluctuating force acting on the mixer shaft.

For baffles tank, for vigorous agitation of thin suspensions, the tank is provided with

baffles which flat vertical strips set radially along the tanks wall as shown in figure 1. Four

baffles are almost always adequate. A common baffle width is 1:10 to 1:12 of the tanks diameter.

For Reynolds number greater than 10,000, baffles are commonly used with turbine impellers and

with on-centerline axial-flow impellers.

In the transition region (Reynolds number, from 10 to 10,000), the width of the baffles

may be reduced, often to ½ of standard width. If the circulation pattern is satisfactory when the

tank is unbaffled but a vortex creates a problem, partial length baffles may be used. These are

standard width and extend downward from the surface into about 1/3 of the liquid volume.

In the region of laminar flow (NRe< 10), the same power is consumed by the impeller

whether baffles are present or not, and they are seldom required. The flow pattern may be

affected by the baffles but not always advantageously. When they are need, the baffles are

usually placed one or two widths radially, to allow fluid to circulate behind them and at same

time produce some axial deflection of flow.

PROCEDURES

General Start-up procedure

1. The power outlet is switched on.

2. All the tightening screws is fastened.

3. The working surrounding area is ensured to be dry and clean.

4. The shaft is lifted up using lifting chain attached to the shaft.

5. The experiment is carried out.

General shut-down procedure

1. Any liquid inside the tank is removed by opening the outlet valve

2. The tank is washed and rinsed to make sure no oil residue after the experiment.

3. The paddle/impeller inside the tank is removed and washed after use.

4. The power outlet is shut down.

Experiment 1

1. The tank is filled with water up to a depth of 30L.

2. Flat paddle is attached with the end of the shaft.

3. A small quantity of plastic pellet is added to the tank.

3. The speed of the impeller is turned up in small increments: 50 rpm, 100 rpm, 150m rpm and

200 rpm. The pellets are seen to swirl around in the water showing flow patterns.

4. The movement of the pellets and the flow pattern is observed and drawn.

5. The procedures are repeated by replacing the flat paddle with other impellers : turbine

impeller and screw propeller.

6. The procedures are also repeated with the baffles fitted in the tank with each flat paddle,

turbine impeller and screw propeller.

Experiment 2

1. The tank filled with coagulant up to a depth of 30 L

2. Flat paddle is attached with the end of the shaft.

3. The speed of the impeller is turned up to 50 rpm and the reading of force is recorded.

4. The speed is then turned up to 100 rpm, 150 rpm and 200 rpm with the force recorded at the

respective speed.

5. Step 3-4 is repeated with the baffles fitted in the tank.

6. The power consumed for each of the speed is calculated.

APPARATUS

Fluid mixing apparatus

Water

Coagulant

Plastic pellets

RESULTS

Flat paddle

Tank

Baffles

Turbine Impeller

Force indicator

Screw propeller

Speed controller

WITHOUT BAFFLES

Flat paddles blade.

Angular speed (rpm)

Angular speed ω (rad-1)

Force F (N) Torque T (Nm) Power W (watts)

50

100

150

200

WITH BAFFLES

Flat paddles blade

Angular speed (rpm)

Angular speed ω (rad-1)

Force F (N) Torque T (Nm) Power W (watts)

50

100

150

200

Formulas:

Power (P) = Torque (T) x Angular Speed ω (rads-1 )

Torque (T) = Force recorded on spring balance (F) x length of torque arm(0.11m)(r)

Torque arm (r) = 0.11m

Angular speed (ω) = N (r.p.m.) x 2π = rads-1

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